Down Syndrome (DS) is a major cause of mental retardation that arises from the presence of three copies (+/+/+) of chromosome 21. The mechanisms of mental retardation in DS have long been studied, but the basic etiology remains unknown. A portion of mouse chromosome 16, among other genes localized to this chromosome, is almost identical to nearly the entire human chromosome 21. Therefore, segmental trisomy 16, (+/+/+, Ts65Dn) is considered an animal model of DS. The gene encoding the G protein-activated potassium channel (GIRK) subunit 2, GIRK2, has been found within the DS critical region of chromosome 21. This is the only gene coding for a channel subunit from the DS critical region that is highly expressed, i.e., GIRK2 protein is synthesized in the hippocampus and cerebellum. The Ts65Dn mouse has three copies of GIRK2 genes (+/+/+). GIRK2 forms a pore on the outer membrane of nerve cells. Ts65Dn mice demonstrate impaired behavior and learning that are associated with abnormal growth of the nerve synapse and synaptic plasticity in the hippocampus and neuropathological changes in the cerebellum. We have carried out fluorescence immunohistochemical studies with a GIRK2 specific antibody on the cerebellum of the DS (trisomy) mouse, control mice, the heterozygous mouse with one GIRK2 gene (+/-), and the heterozygous/trisomy mouse with 2 GIRK2 genes (+/+/-). Initial immunohistochemical data revealed that the GIRK2 protein was present in all 4 groups of mice. However it was a higher expression (increased fluorescence intensity) in the DS mouse. Unipolar brush cells (UBC) are recently described cells in the cerebellum granule cell layers (IX, X, I) known as the vestibulo-cerebellum. They were identified as unique cells using an antibody known as calretinin which was developed in our laboratory (Abbott and Jacobowitz, 1995; Floris et al., 1994; Arai et al., 1991). They have recently been described as excitatory interneurons that contain glutamate as a neurotransmitter. The UBCs give rise to an extensive system of mossy fibers that innervate granule cells in the cerebellum (Nunzi et al., 2003). We have found that the DS (trisomy, +/+/+) mouse has an increased intensity of fluorescence in the UBC cells while the heterozygous mouse has a marked decrease in the intensity of fluorescence which suggests that there is a reduction in the concentration of the GIRK2 protein in the heterozygous mouse. It appears that the level of expression of the GIRK2 in UBC cells is directly related to GIRK2 gene number. The molecular layer of the cerebellum contains specialized cells known as Bergmann glia. These glia project long fibers that extend from the Purkinje cell layer to the surface of the molecular layer. It is believed that the Bergmann glial fibers invest the Purkinje cell processes with a protective and/or insulating sheath. In this view, the Bergmann glia is a permanent satellite of the Purkinje cell. Numerous Bergmann glia fibers were observed in the molecular layer of the cerebellum of the DS (+/+/+) mice but were markedly reduced in the controls (+/+). No Bergmann glia was noted in the heterozygous mice (+/-). This suggests that at least 2 copies of the GIRK2 gene are necessary to have a significant expression of GIRK2 in Bergmann glia. In summary, these preliminary findings reveal that the GIRK2 protein is overexpressed in the DS mouse in comparison to GIRK2 expression n the normal control mice (+/+) and underexpressed in the heterozygous trisomy (+/+/-) mouse. These findings are consistent with the gene number effect on the GIRK2 protein expression which would have significant physiological consequences on functionality of neuronal networks in specific brain regions.